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For some reason, even among the mega fauna, everything was HUGE here.[/QB]
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Mass homeothermy. Bigger beast = lower surface area to body-mass ratio = easier to stay warm. Most commonly seen in warm-blooded creatures. Larger size also means much greater resistance to predators, and the ability to prey upon larger food sources (which adds the benefit of having to hunt less often--one kill can feed the family for a week).
That doesn't explain the strange concentration that you describe in Beringea, though. Perhaps food and other resources were extremely plentiful, sparking an "arms race" of ever-bigger beasties. The opposite certainly occurs when resources are scarce--Shetland ponies, to take a modern-day example.